feat(markdown-editor): implement wysiswyg markdown editor with live preview

- Build Go backend with Cobra CLI and REST API
  - CRUD operations for markdown files (GET, POST, PUT, DELETE)
  - File storage with flat .md file structure
  - Comprehensive logrus logging with JSON format
  - Static asset serving for frontend

- Build React/TypeScript frontend with Tailwind CSS
  - Markdown editor with live GFM preview
  - File management UI (list, create, open, delete)
  - Theme system (Dark/Light/System) with persistence
  - Responsive design (320px mobile, 1920px desktop)

- Add comprehensive test coverage
  - Backend: API, storage, and logger tests (13 tests passing)
  - Frontend: Editor and App component tests

- Setup Nix development environment with Go, Node.js, and TypeScript

backend/internal/storage/storage.go

@@ -0,0 +1,74 @@
+package storage
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/markdown-editor/pkg/logger"
+)
+
+type Storage struct {
+	dataDir string
+}
+
+func NewStorage(dataDir string) *Storage {
+	// Ensure data directory exists
+	if err := os.MkdirAll(dataDir, 0755); err != nil {
+		logger.Fatalf("Failed to create data directory: %v", err)
+	}
+	return &Storage{dataDir: dataDir}
+}
+
+func (s *Storage) ListFiles() ([]string, error) {
+	files, err := os.ReadDir(s.dataDir)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read directory: %w", err)
+	}
+
+	var mdFiles []string
+	for _, file := range files {
+		if !file.IsDir() && strings.HasSuffix(file.Name(), ".md") {
+			mdFiles = append(mdFiles, file.Name())
+		}
+	}
+	return mdFiles, nil
+}
+
+func (s *Storage) GetFile(filename string) (string, error) {
+	path := filepath.Join(s.dataDir, filename)
+	content, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return "", fmt.Errorf("file not found: %s", filename)
+		}
+		return "", fmt.Errorf("failed to read file: %w", err)
+	}
+	return string(content), nil
+}
+
+func (s *Storage) SaveFile(filename, content string) error {
+	path := filepath.Join(s.dataDir, filename)
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		return fmt.Errorf("failed to write file: %w", err)
+	}
+	return nil
+}
+
+func (s *Storage) DeleteFile(filename string) error {
+	path := filepath.Join(s.dataDir, filename)
+	if err := os.Remove(path); err != nil {
+		if os.IsNotExist(err) {
+			return fmt.Errorf("file not found: %s", filename)
+		}
+		return fmt.Errorf("failed to delete file: %w", err)
+	}
+	return nil
+}
+
+func (s *Storage) Exists(filename string) bool {
+	path := filepath.Join(s.dataDir, filename)
+	_, err := os.Stat(path)
+	return err == nil
+}

backend/internal/storage/storage_test.go

@@ -0,0 +1,141 @@
+package storage
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func setupTestStorage(t *testing.T) *Storage {
+	tempDir := t.TempDir()
+	return NewStorage(tempDir)
+}
+
+func TestListFiles(t *testing.T) {
+	storage := setupTestStorage(t)
+	
+	// Create test files
+	content := "Test content"
+	testFiles := []string{"test1.md", "test2.md", "notes.md"}
+	
+	for _, filename := range testFiles {
+		path := filepath.Join(storage.dataDir, filename)
+		if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+			t.Fatalf("Failed to create test file: %v", err)
+		}
+	}
+	
+	files, err := storage.ListFiles()
+	if err != nil {
+		t.Fatalf("Failed to list files: %v", err)
+	}
+	
+	if len(files) != 3 {
+		t.Errorf("Expected 3 files, got %d", len(files))
+	}
+	
+	expected := map[string]bool{"test1.md": true, "test2.md": true, "notes.md": true}
+	for _, file := range files {
+		if !expected[file] {
+			t.Errorf("Unexpected file: %s", file)
+		}
+	}
+}
+
+func TestGetFile(t *testing.T) {
+	storage := setupTestStorage(t)
+	
+	filename := "testfile.md"
+	content := "# Test Heading\n\nTest content."
+	path := filepath.Join(storage.dataDir, filename)
+	os.WriteFile(path, []byte(content), 0644)
+	
+	fileContent, err := storage.GetFile(filename)
+	if err != nil {
+		t.Fatalf("Failed to get file: %v", err)
+	}
+	
+	if fileContent != content {
+		t.Errorf("Expected content %q, got %q", content, fileContent)
+	}
+}
+
+func TestGetFileNotFound(t *testing.T) {
+	storage := setupTestStorage(t)
+	
+	_, err := storage.GetFile("nonexistent.md")
+	if err == nil {
+		t.Error("Expected error for non-existent file")
+	}
+}
+
+func TestSaveFile(t *testing.T) {
+	storage := setupTestStorage(t)
+	
+	filename := "newfile.md"
+	content := "# New File\n\nContent here."
+	
+	err := storage.SaveFile(filename, content)
+	if err != nil {
+		t.Fatalf("Failed to save file: %v", err)
+	}
+	
+	// Verify file was saved
+	path := filepath.Join(storage.dataDir, filename)
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		t.Error("File was not saved")
+	}
+	
+	// Verify content
+	storedContent, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("Failed to read file: %v", err)
+	}
+	if string(storedContent) != content {
+		t.Error("File content does not match")
+	}
+}
+
+func TestDeleteFile(t *testing.T) {
+	storage := setupTestStorage(t)
+	
+	filename := "todelete.md"
+	content := "To be deleted."
+	path := filepath.Join(storage.dataDir, filename)
+	os.WriteFile(path, []byte(content), 0644)
+	
+	err := storage.DeleteFile(filename)
+	if err != nil {
+		t.Fatalf("Failed to delete file: %v", err)
+	}
+	
+	// Verify file was deleted
+	if _, err := os.Stat(path); !os.IsNotExist(err) {
+		t.Error("File was not deleted")
+	}
+}
+
+func TestDeleteFileNotFound(t *testing.T) {
+	storage := setupTestStorage(t)
+	
+	err := storage.DeleteFile("nonexistent.md")
+	if err == nil {
+		t.Error("Expected error for non-existent file")
+	}
+}
+
+func TestExists(t *testing.T) {
+	storage := setupTestStorage(t)
+	
+	filename := "exists.md"
+	path := filepath.Join(storage.dataDir, filename)
+	os.WriteFile(path, []byte("content"), 0644)
+	
+	if !storage.Exists(filename) {
+		t.Error("File should exist")
+	}
+	
+	if storage.Exists("nonexistent.md") {
+		t.Error("Non-existent file should not exist")
+	}
+}